This invention relates to a disc brake.
A disc brake of the generic type is known from DE (German Patent Application) No. 32 41 164 C2. This known disc brake has a hydraulic actuating device consisting of a brake cylinder and of a brake piston arranged axially displaceably in the brake cylinder. Brake piston and brake cylinder are sealed relative to each other by means of a sealing ring. The sealing ring is arranged in a surrounding annular groove inside the brake cylinder and frictionally rests for sealing purposes on an outside surface area of the brake piston. The sealing ring does not only have the duty to seal the inside of the brake cylinder; it also helps adjust a clearance between the friction surfaces of the brake pads and the brake disc after the brake has been applied.
When the brake is actuated hydraulic pressure is applied to the brake piston which thus moves axially out of the brake cylinder. The friction between the sealing ring and the brake piston causes the sealing ring which rests on the outside surface area of the brake piston to be elastically deformed during axial displacement of the brake piston. If the hydraulic pressure is removed, the sealing ring will return to its original form, thus moving the brake piston by a small degree to the brake cylinder. A brake pad resting on the brake piston follows the axial displacement of the brake piston, with its friction surface being lifted away from the brake disc and thus a clearance being adjusted.
In general, the size of the adjusted clearance depends on the condition of the sealing ring and of the annular groove which accommodates the sealing ring, particularly on the cross-sectional shape of the annular groove. There exists a further undesired dependence on the hydraulic pressure prevailing in the brake cylinder during brake application. It is a general aim always to keep an invariably constant clearance after any brake application, no matter what hydraulic pressures will appear during brake application. In order to achieve this object there have already been many suggestions concerning different cross-sectional shapes of the annular groove. However, a generally valid measure which will reliably perform in all applications has not yet been found. For instance, the disc brake known from DE 32 41 164 C2 features a step in the transitional area between a groove wall and the cylinder surface of the brake cylinder.